Gamma-aminobutyric acid (GABA) is a non-protein constituent amino acid, widely found in animals and plants. Particularly in high-degree animals, GABA is known as an important neurotransmitter inhibitor, and in also known to promote blood circulation and increase oxygen supply to accelerate brain cell metabolism in the brain. Also, GABA is not only involved in prolactin secretion and growth hormone regulation, but it also lowers blood pressure and has pain relieving effects, thereby making itself a pharmacologically interesting compound.
It has been known that GABA accelerates sodium ion discharge through urine, so that it may lower blood pressure to prevent hypertension which may be caused from excessive consumption of salts.
According to previous reports, GABA can also accelerate alcohol metabolism, activate kidney functions, improve liver functions, and prevent obesity.
GABA is a kind of natural amino acid, which can also be generated during fermentation by lactobacillus. GABA is represented by the molecular formula: H2NCH2 CH2COOH and has the melting point of 203° C. It is highly soluble in water. When ingested by humans it is not accumulated in the body; namely, it is degraded without side effects.
Glutamate decarboxylase (GAD) is involved in the mechanism of GABA generation by fermentation of lactobacillus. The extracellular accumulation of metabolites in the late proliferation stage breaks the balance between extracellular and intracellular hydrogen ions (H+), therefore a counter action to maintain the balance has to occur, during which GABA is generated. That is, when a kind of extracellular amino acid glutamate transfers inside cells, the intracellular hydrogen ions (H+) are consumed by the carboxyl group of the glutamate being substituted for an intracellular accumulated hydrogen ion (H+) to generate carbon dioxide (CO2). Thereby, GABA is generated during the above process.
A previous study on increase of GABA content using soybeans reported that when soybeans were anaerobically treated with a gas such as nitrogen dioxide, GABA content increased about 7.4 fold (156.8∇/100 g) compared with a control group (Mitsuaki et al., Gamma-aminobutyric acid Accumulation in Bean Sprouts (Soybean, Black Gram, Green Gram) Treated with Carbon Dioxide. Nippon Shokuhin Kogyo Gakkaishi. 36(11):916-919. 1989). However, such modified atmosphere method is very troublesome and investment for facilities are required.
Recent studies show that the glycosides included in soybeans or various fermented products from fermentation by microorganisms have physiological activities. Thus, Korean traditional fermented pastes came into the spotlight as health foods. Fermented soybeans (Meju in Korean) are used as major or sub-material for Korean traditional fermented paste, which has been an important protein source since long times ago. However, since this Korean fermented paste has a high salt content, it is a major cause of hypertension and various adult diseases mainly caused by excessive intake of salt.
The conventional method for producing fermented soybeans is as follows: soybeans are washed, soaked in water, steamed, and ground. Then, the ground soybeans are molded into various shapes such as a hexahedron, a cylinder, a sphere, etc, which are fermented by a natural microorganism. Lactobacillus is a very fastidious microorganism that grows better under anaerobic conditions than under aerobic conditions, and requires nutrients such as various amino acids, vitamins, salts and specific peptides, etc. Therefore, abundant nutrients contained in soybeans are sutable to grow lactobacillus and the inside of fermented soybeans naturally provide anaerobic conditions.
Also, organic acids generated by the metabolism of lactobacillus in fermented soybeans reduces the pH of fermented soybeans, as a result, which provide a suitable environment for GAD to be activated. In addition, such organic acids can prevent the contamination of other bacteria during the fermentation of the soybean in the air, thereby preparing the fermented soybeans of regular quality. Nevertheless, a method of manufacturing fermented soybeans using lactobacillus together with glutamate has not yet been proposed.